Let's say that you're going to sharpen your knife once per quarter as a simple achievable goal that will get you out of your comfort zone. What should you look into so your learning is as diverse and useful as possible?
Books - One of the oldest ways to learn about any craft is books. It's not a surprise and it's something that you're comfortable with, so it's a great goal for quarter one. Read a book about your craft, perhaps it's about a particular part of your craft. If you write read a book about writing romance, or writing detective stories, or writing in first person. Any of these are great ways to expand your knowledge and sharpen the knife.
Software - Now, bare with me here because if you're working with oil paints there aren't that many software's that are going to help you with your craft. Instead perhaps this should be called 'try a new tool'. Still, if you're a writer then I don't mean a writing tool! I mean a tool that will help you with the physical writing, not first person or third person or 'hanging a lantern on it'. Try Aeon Timeline, try Scrivner, try 4thewords. All three of them allow for a one month free trial and all three of them are vastly useful in different parts of writing.
Tracing - Most who draw start to learn by tracing over the art of another. It's a wonderful way to learn about proportions and what goes where and why someone did what they did. Therefore looking at a new style of writing or drawing or music and trying it out by doing a simple trace can be a great way to expand your craft! Be careful not to try to make money off of a direct trace of someone else's work, but as an exercise it's vastly useful.
Classes - A class where they give you material and guidance towards learning a new skill including some amount of trying to implement said skill. I find this to be a wonderful way to learn new skills because there are additional people learning with me at the same time. Just like with NaNo when people are taking on the same challenges it seems that much easier to overcome, and if there are difficulties with trying to learn there are many people who can help at any time.
I would absolutely recommend a combination of all of these approaches, kinda like using different whet stones in order to get the finest edge on your blade.
100 Squats per Day
Read 52 Books